[QTI] Formulas using a LaTeX matrix, alignment or \boxed construct don't publish as MathML #6347

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Target branch: unstable

Observed behavior

• Publishing converts every legacy exercise to a QTI package (QTIExerciseGenerator). The conversion parses each formula's MathML into the pydantic MathML models.
• Those models reject MathML that latex2mathml produces for common LaTeX: \begin{matrix}, \begin{pmatrix}, \begin{cases}, \begin{array}, \boxed, \substack.
• The error reaches recurse_nodes uncaught, so the whole channel publish fails, not just the one item.
• One cause: Mtable is not a presentation element, so Mrow refuses it. menclose has no model at all.
• The same MathML in the MathML namespace passes validate_qti_item.
• Same rule broken as [QTI] A QTI item containing an XML comment fails the channel publish #6229 and [QTI] A QTI item with a DOCTYPE entity reference fails the channel publish #6234 (native items): one item's error fails the publish instead of skipping that item. Their fixes don't reach the legacy conversion.
• latex2mathml 3.78.1 writes the alignment & of aligned, align, alignat, gathered, eqnarray, rcases, dcases unescaped (<mi>&</mi>), so _convert's ET.fromstring raises ParseError. On unstable this aborts the publish too; with the [QTI] - The formulas are not shown in Kolibri #6327 fix the formula publishes as $$…$$ text instead, in legacy and QTI editor (<span data-latex>) items alike.

Errors and logs

pydantic_core._pydantic_core.ValidationError: 2 validation errors for Mrow

ValueError: No registered class found for tag: menclose

xml.etree.ElementTree.ParseError: not well-formed (invalid token)

User-facing consequences

• A channel with one legacy question holding a matrix, a piecewise function or a boxed answer cannot be published at all.
• With the [QTI] - The formulas are not shown in Kolibri #6327 fix, learners see the raw LaTeX source in place of a formula using an alignment environment.

Steps to reproduce

1. On unstable (3857a5e), create an exercise with a single-choice question whose text is $$\begin{pmatrix}1\\2\end{pmatrix}$$.
2. Publish the channel.
3. The publish fails with the ValidationError above; $$\boxed{x}$$ fails with the ValueError; $$\begin{aligned}a&=b\\c&=d\end{aligned}$$ fails with the ParseError.

Acceptance Criteria

• A legacy question containing each of \begin{matrix}, \begin{pmatrix}, \begin{cases}, \begin{array}{cc}, \boxed{x}, \substack{a\\b} publishes, and its QTI item passes validate_qti_item.
• A formula using each of aligned, align, alignat, gathered, eqnarray, rcases, dcases with & publishes as MathML in the MathML namespace with its application/x-tex annotation, from legacy and QTI editor items, and the item passes validate_qti_item.
• A formula the QTI item cannot carry costs at most that item, never the channel publish.

AI usage

Found by Claude Code while planning #6327: it probed latex2mathml output against the MathML models and the item XSD on unstable, and drafted this issue.